Episode 8 is being aired on TV this week, but it’s the last episode of Book 3 that is going to be premiered on TV. Episodes 9-13 are going to be what is called “digitally released”, which means that Nick.com and other websites like Amazon, etc. will have them up on the release dates (hopefully) and that is where we will have to watch them. It is said that this book of Korra has had some pretty low ratings (which I believe is due to the lack of promotion by Nickelodeon), so that is why it is getting pulled from the TV program.
At least we’re guaranteed Book 4. Hope I was able to clear it up for you anon! (I had no clue what was going on until earlier today)